One of the problems limiting the development of high performance ceramic materials is a very basic one, i.e., it is often difficult, if not impossible to mix two or more different particulate materials and obtain a homogenous composite mixture. The differences between particles could be compositional, geometrical, surface chemical, etc. Processing difficulties are exacerbated for fine particles (micron and submicron sized), which are of principal interest in the manufacture of high performance ceramic and metallic parts, formed to net shape. This requires extraordinary particulate and process control, exceeding that which is currently practiced as the state of the art. Effective homogenization and maintenance is particularly difficult for dry mixed fine particulate materials and engineers have turned toward wet methods for success. Matijevic (1986) "Colloidal Science of Composite Systems" in The Science of Ceramic Chemical Processing (L. L. Hench, et al., Editors) Wiley Interscience Publ., pg. 463 (1986) discusses the difficulties involved in the wet preparation of fine composite particulate systems and he sites three potential techniques: "(1) homogeneous precipitation from solution of mixed chemical composition, (2) precipitation of solid(s) in a dispersion already containing one or more kinds of performed particles and (3) mixing dispersions of different particles". Wet mixing of dispersions of different particles is state of the art in the coarse particulate industries and is currently being investigated in the fine particle industries. The other two techniques are still in their infancy.
While wet mixing methods have demonstrated that wet processing can yield microscopically homogeneous composites (Carlstrom and Lange, "Mixing of Flocced Suspension" J. Am. Ceram. Soc. v67(8): c169-170, 1984; Aksay et al. "Uniformity of Al.sub.2 O.sub.3 -ZrO.sub.2 Composites by Colloidal Filtration" J. Am. Ceram. Soc. v66(10): c190-193, 1983), wet processing methods have been limited to dilute systems (typically 20-30 volume percent solids). In order to effectively and efficiently carry out the sintering process, it is preferred that the particle packing in the dried, prefered body by appproximately 50 volume percent solids and preferably 60 volume percent solids. To achieve these prefered particle concentrations, the composite suspension must undergo a dewatering-consolidation step, generally known as the forming process. Inherent in the forming process are the establishment of gradients and non-uniformities within the sample as the particles consolidate and the pore fluid is extracted. This can lead to gross particle packing, compositional inhomogenities and non-controllable drying shrinkage resulting in poor process control and reproducibility. Being able to mix dissimilar materials at high solids loading greatly would reduce the potential for microstructural inhomogenities and give the process engineer greater control of the net shape forming process. Although this maximum solids fraction can be higher if the particles do not have a narrow size range, narrow sized particles are precursors to uniform ceramics having uniformly reproducible properties, such as drying and firing shrinkage for net shape manufacture, and for high performance applications.
For example, it is difficult to mix partially stabilized colloidal zirconia with colloidal alumina in aqueous suspension. In conventional aqueous colloidal dispersions, the alumina particles (matrix phase) have a negative surface charge. When the partially stabilized zirconia (dopant), which has a positive surface charge, is added, the mixture hetero-coagulates. The use of dispersants helps, but is restricted due to the different surface chemistries of the colloidal phases. The coagulation imposes a rheological limit for the amount of dopant and solids that can be added. Further, due to poor mixing, the resultant mixture is not homogenous.
This problem is not limited to the addition of colloidal suspensions or powders. A growing trend in ceramic engineering is to develop materials with increased fracture toughness by the addition of ceramic fibers. The fiber may be made out of the same material as the matrix, but is often made from a different, stronger material. Fibers improve toughness by preventing tiny cracks which appear in stressed material from developing into large cracks, which ultimately cause the material to shatter. However, to be effective, the fibers should be uniformly distributed throughout the matrix. Similar particle interactions as the ones described above make uniform distribution a difficult task.
The addition of whiskers to a material is another method of imparting greater fracture toughness, stiffness, and tensile strength to ceramic products. Whiskers differ from fibers in that whiskers are mono-crystalline whereas fibers are polycrystalline. Whiskers are much thinner and shorter than fibers. They tend to be stronger than fibers, as well. Like fibers, they need to be uniformly dispersed throughout the ceramic in order to impact optimum properties, but due to particle-particle interactions, this is difficult to achieve.
A higher volume percent of solids would result in less shrinkage upon drying and sintering, superior green strength, and a reduction in the probability of a large defect or void in particle packing. In addition, faster processing results from the reduced volume fraction of liquid.
Accordingly, there exists a need for a composite system for maintaining a maximum solids content of narrow size distribution in suspension, thereby maximizing green strength and minimizing shrinkage, while still maintaining the components in a flowable or pourable state.